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Neuro > Vision > Flashcards

Vision Flashcards

1
Q

Layers of the retina (light path)

A

Retinal ganglion layer
Inner plexiform layer
Inner nuclear layer
Outer plexiform layer
Outer nuclear layer
Photoreceptor layer
Pigment epithelium

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2
Q

Steps of image processing

A

Phototransduction
Synaptic transmission
Nervous activity
Synaptic transmission
Nervous impulse
Processing in the LGN
Nervous impulse
Image integration

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3
Q

Photoreceptor distribution and wavelength

A

Short cones - 420
Rods - 498
Medium 534
Long - 564

Don’t exclusive detect one distinct wavelength but a range
Each cone can detect around 100 shades of its specific colour

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4
Q

Red-green colourblindness

A

4 types:
Deuteranomaly - most common. Green seems more red. Mild
Protanomaly - makes red look more green and less bright. Mild
Protanopia and deuteranopia both make you enable to tell the difference between red and green at all

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5
Q

Photoreceptor structure

A

Membrane shelves lined with rhodopsin or colour pigment (outer segment)
Mitochondria (inner segment)
Outer limiting membrane
Nucleus
Synaptic body

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6
Q

11-cis-Retinaldehyde converts to what in light

A

All trans retinaldehyde

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7
Q

Phototransduction (dark phase)

A
  1. Guanylate cyclase (GC) is constitutively active and cGMP is constantly being formed in the outer segment (OS).
  2. cGMP opens cation channels (CNGA) in the OS membrane (generating a ‘dark current’) and the OS depolarises.
  3. Depolarisation spreads to inner segment (IS) and VGCCs open in presynaptic membrane.
  4. Vesicles containing Glutamate fuse with the presynaptic membrane (Ca2+ dependent).
  5. Glutamate enters the synaptic cleft.
  6. mGLUR6 receptors on the Bipolar cell activate (GPCR which links to Gαo – inhibitory G-protein).
  7. Gαo inhibits TRPM1 in ON bipolar cell membrane – hyperpolarisation - ON bipolar cell switched off
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8
Q

Phototransduction - Light phase

A
  1. Light causes Opsin to isomerise retinal and activate Transducin, which in turn activates a phosphodiesterase (PDE).
  2. PDE breaks down cGMP
  3. CNGAs close (reduced ‘dark current’) and the OS hyperpolarises.
  4. Hyperpolarisation spreads to the IS and VGCCs close in presynaptic membrane.
  5. Glutamate release is decreased.
  6. mGLUR6 receptors inactivate.
  7. Gαo stops inhibiting TRPM1.
  8. TRPM1 channels open – Na+ enters
  9. ON Bipolar cell depolarises.
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9
Q

Phototransduction is …. To activate but ….. to recover because….

A

Fast to activate slower to recover was the photosensitive pigment needs to be re-built and the cascade reset to terminate the stimulation of bipolar cells

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10
Q

Bipolar cell activity

A

ON bipolar cells hyperpolarise in the dark (due to glutamate release from photoreceptors onto mGLuR6 receptors) but OFF bipolar cells depolarise (different receptors (iGLuR2) for glutamate means different effect) – important for dark light contrast!
• In the light ON bipolar cells depolarise (due to reduced glutamate release) and OFF bipolar cells hyperpolarise.
• Increases in light intensity are transmitted in ON bipolar cells while decreases in light intensity are transmitted by OFF bipolar cells
Horizontal cells (H) also interact with ON bipolar cells

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11
Q

Bipolar cell activity - receptive field

A

Light directly falling on one photoreceptor switches off adjacent bipolar cells due to horizontal cell mediated hyperpolarisation
• Lateral inhibition
• Important for image processing
• Improves visual acuity such that photons of light don’t stimulate lots of adjacent photoreceptors – reduces blurry edges

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12
Q

Bipolar cell to ganglion cell synaptic activity

A

All bipolar cells when stimulated by photoreceptors transfer the graded signal to a neighbouring retinal ganglion cell (RGC) via glutamate release.
• Postsynaptically iGLUR (iGLuR1) receptors in the RGCs cause depolarisation and action potential is stimulated when the threshold for Na+ channel opening is reached in the axon of the RGC

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13
Q

Retinal ganglion cell (RGC) activity

A

Transmitter same whether bipolar cell is ON or OFF
On bipolar to ON RGC and OFF to OFF

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14
Q

Describe myelination of RGC

A

RGC axons in the retina aren’t myelinated but become myelinated in the optic nerve due to oligodendrocytes
Abnormal myelination increases the blind spot

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15
Q

2 categories of RGCs

A

Based on diameter of axons
Based on their thalami’s projections and functions

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16
Q

What are the retinal ganglion cell types (diameter of axons)

A

• W-ganglion - small, 40% of total, broad dendritic fields in retina, excitation from rods, detect direction movement anywhere in the field.
• X-ganglion - medium diameter, 55% of total, small dendritic field, colour vision. Sustained response.
• Y- ganglion cells - largest, 5%, very broad dendritic field, respond to rapid eye movement or rapid change in light intensity. Transient response.

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17
Q

What are the RGC types based on projections and functions

A

• Midget cells or M RGCs (Magnocellular or M pathway) – rods – movement & contrast
• Parasol cells or P RGCs (Parvocelluar or P pathway) – M&L cones (green & red)
• Bistratified cells or K RGCs (Koniocellular or K pathway) - S cones (Blue)
• Intrinsic Photosensitive ganglion cells or ipRGCs - control of pupil diameter, circadian rhythm
• Other ganglion cells projecting to the superior colliculus - eye movements

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18
Q

What happens in the optic chiasma

A

Neuronal crossover

19
Q

Describe thalamus nuclei

A

•P&K RGCs project to the Thalamus (Lateral Geniculate Nuclei) - essential for normal visual perception
• But also other RGCs (eg ipRGCs) project to the hypothalamus, superior colliculus & pretectum
• Reflex orientation to visual stimuli, stabilization of gaze, circadian rhythms and control pupil diameter

20
Q

Describe pupillary responses and the protectable nucleus

A

Light perceived by specialised RGCs called intrinsic photosensitive RGCs
Overtime melanoma in is all isomerise and no new signals transmitted - less signals to the fish - pupils dilate = maintains enough light to photoreceptors to maintain some sight

21
Q

Lateral geniculate nucleus and visual perception

A

RGCs project to the LGN - essential for normal midget RGCs mostly synapse in the mango cellular layers of LGN
Parasol only synapse in parvcellualr layers of LGN
Bistratified or K RGCs mostly synapse in Koniocellular layers between the individual layers of the magno and Parvo

22
Q

What is importan in signal modulation in LGN

A

Feed-forward, feedback and lateral inhibition

23
Q

LGN may determine….

A

Eyedominance

24
Q

RGC project to the ……

A

Thalamus (LGN)

25
Q

Photoreceptor -> RGC -> layer of LGN

A

Rod-> M -> magnocellular
M and L cones -> P -> parvocellular
S cone -> K -> Koniocellular

26
Q

RGCs also project to the …

A

Hypothalamus, superior colliculus and pretectum

27
Q

The 2 theories of colour vision

A

Trichromatic and opponent process

28
Q

describe the trichromatic theory

A

The retinas 3 types of cones are preferentially sensitive to blue, green and red

29
Q

Describe the opponent process theory

A

Visual system interprets colour in an antagonistic way: red vs green, blue vs yellow, black vs white

30
Q

Colour perception depends on…..

A

Bipolar cells

31
Q

Describe colour perception and bipolar cells (stars and stripes)

A

Continued activation of blue and green photoreceptors by cyan in the stripes switches off (desensitizes) GvsR and BvsY bipolar cells
• RvsG bipolar cells become highly available so when the cyan disappears and the brain perceives red stripes as nothing is blocking activity of the RvsG bipolar cells (as lateral inhibition is lost) and they can then respond to the wide range of light wavelengths in white light
White stars on a Blue background
• YvsB bipolar’s desensitize over time as do GvsR and RvsG because yellow light stimulates both receptors – switch off the yellow signal and blue is perceived by the brain (BvsY bipolar cells) when white light is viewed.
• Black light doesn’t stimulate the rods so the rods are available to process the white as white.

32
Q

Colour perception is manipulated by the …..

A

Visual cortex
CHROMATIC ADAPTATION

33
Q

Disorders of higher visual processing (ventral)

A

Cerebral achromatopsia
Cerebral dyschromatopsia
Colour anomia or agnosia
Visual agnosia
Prosopagnosia
Alexia

34
Q

Cerebral achromatopsia

A

Complete absence of colour perception

35
Q

Cerebral dyschromatopsia

A

Impaired colour perception (hues and saturation)

36
Q

Colour anemia or agnosia

A

Can discriminate colours accurately but cannot name them

37
Q

Visual agnosia

A

No longer recognise previously familiar objects and cannot learn to identify new objects by sight alone

38
Q

Prosopagnosia

A

Impaired ability to recognise familiar faces or to learn o recognise new faces

39
Q

Alexia

A

Can write but not read well
Disconnection of the visual input in both hemifields from language areas in the left hemisphere

40
Q

Disorders of higher visual processing - dorsal pathways

A

Cerebral akinetopsia
Billings syndrome
Blindsight

41
Q

cerebral akinetopsia

A

A selective impairment in motion perception

42
Q

Balint’s syndrome

A

Deficit in attention

43
Q

Blindsight

A

Impaired conscious awareness of visual stimuli

44
Q

What is processed in the visual cortex

A

Colour vision, depth perception, orientation, what and where

Neuro (26 decks)

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